Anatomy
The femoral nerve is one of the major branches of the lumbar plexus. The femoral
nerve is consistently lateral to the femoral artery, deep to the fascia iliaca and
superficial to the iliopsoas muscle. The anterior approach to block the femoral nerve
at the groin (inguinal region) is most commonly performed for knee surgery.

FA = femoral arteryFN = femoral nerveFV = femoral vein

Picture taken from Colour Atlas of Anatomy: A photographic study of the human body (3rd edition)

After skin and transducer preparation, place a transducer with the
appropriate frequency range (10-12 MHz) along the inguinal crease.
If the femoral artery and nerve are deep (> 4 cm, use a 7 MHZ transducer).

Perform a systematic anatomical survey from medial to lateral and
superficial to deep.

The femoral nerve is generally easy to locate in this region.

First, identify the femoral artery. If the image shows more than 1
artery, scan more proximally (cephalad) to visualize the artery
before the profunda femoris artery branches off.

The femoral vein is medial to the artery. The vein may not be visible
until the transducer pressure on the skin is lessened.

Deep to the femoral vessels is the iliopsoas muscle bulk.

The femoral nerve is often found within a triangular hyperechoic region,
lateral to the femoral artery and superficial to the iliopsoas muscle.

The femoral nerve may be quite thin and flat in this region as the nerve
fans out into multiple branches.

Note the fascia iliaca (a hyperechoic line) superficial to the femoral
nerve and its branches.

Inguinal lymph nodes also appear hyperechoic and may be confused with
the nerve in the short axis view. To distinguish the two, scan
proximally and distally in this region. A nerve is a continuous
structure that can be traced while a lymph node is not and can be
seen only in a discrete location.

Needle Insertion ApproachIn Plane Approach

The in plane approach is also commonly used for femoral nerve block by
aligning the block needle with the ultrasound beam.

With this approach, the needle shaft and tip can be visualized distinctly
but it may take a longer time to align the needle with the beam
compared to the out of plane approach.

Insertion of a block needle over the left inguinal region using the in plane approach

In plane needle approach showing needle in contact with the femoral nerve

Arrows = block needleFA = femoral arteryFV = femoral vein

In plane needle approach showing needle in contact with the femoral nerve

Ultrasound guided femoral nerve block is considered a BASIC skill level
block because this is a superficial block.

Insert a 5 cm 22 G insulated needle perpendicular to the transducer and
the ultrasound beam. In this case, only the cross section of the
needle shaft (a white dot) may be observed during needle advancement.

It can be technically challenging to track the location of the needle
tip during needle insertion without an echogenic tip design. Move
the needle tip slightly from side to side or in and out to bring
the tip into view.

Injection of a small amount of fluid e.g., D5W will expand the femoral
triangle and the hypoechoic fluid collection can bring the hyperechoic
nerve and the fascia iliaca into view.

Injection of D5W (1-5 mL) will also intensify the motor response to
nerve stimulation.

The posterior division of the femoral nerve which innervates the
quadriceps muscles is most commonly located on the lateral aspect of
the femoral triangle. It is therefore recommended to first point the
needle towards the lateral aspect of the femoral triangle under
ultrasound guidance.

Once satisfied with needle placement, inject 20-30 mL of local anesthetic
for surgical anesthesia or postoperative analgesia. Observe "sheath"
distention and a hypoechoic ring of local anesthetic solution around
the hyperechoic nerve structures.

Scan proximally and distally to assess the extent of local anesthetic
spread.

Clinical PearlsNerve Localization1. Inguinal Lymph Nodes vs. the Femoral Nerve
The inguinal lymph nodes may resemble the femoral nerve in cross section with a
single level scan. It is therefore important to scan proximally and distally at
the inguinal region and trace the course of the femoral nerve. In contrast, the
inguinal lymph nodes are discrete superficial structures.

2. Aberrant Femoral Nerve Location
It is important to scan proximal and distal to the inguinal region. The posterior
division of the femoral nerve may be found above the iliopsoas muscle far lateral to the femoral artery.

Aim to place the needle and a 20 G catheter within the femoral triangle
deep to the fascia iliaca.

Once the block needle is in contact with the femoral nerve (+/- nerve
stimulation), inject 5-10 mL of local anesthetic or D5W (if nerve
stimulation is desired) to distend the perineural space.

It is important to recognize improper injection that is outside the
perineural space.

The catheter is often inserted without real time ultrasound guidance
unless an assistant is available to hold the ultrasound transducer
in place while the principal operator uses one hand to hold the
needle and the other hand to thread the catheter.

Aim to thread the catheter 3-5 cm beyond the needle tip.

It may be difficult to visualize local anesthetic spread at the time of
injection when the catheter tip is deep within the pelvis beyond the
inguinal ligament.

Catheter advancement may not necessarily follow the course of the
femoral nerve because there are several channels in the perineural
compartment (1, 2, 3 and 4 as shown in the figure below).

This highlights the theoretical advantages of using a stimulating
catheter to ensure proper perineural catheter placement.

Observe the hyperechoic catheter tip (arrow) location and local anesthetic spread
at the time of injection through the catheter.
It is possible to see several hyperechoic dots in the local anesthetic collection
indicating coiling of the catheter.